This invention relates to an electron beam inspection system using a field emission electron gun and a method and apparatus for measuring and correcting for the effect of beam current noise on the scanned electron image.
In scanning electron beam inspection systems, defects are detected by comparing the signals from corresponding image pixels in the tested chip (die) and the reference standard. The reference standard may be an electronic database in which case the inspection is called die-to-database inspection, or the reference may be another test die, in which case the inspection is called die-to-die inspection. A defect is found when the signals between the die and the reference differ by more than a given detection threshold.
Field emission electron guns provide a suitable electron source for such scanning electron beam inspection systems. In a field emission electron gun, a voltage potential is applied between an emitter tip and the target. The electrostatic field present at the emitter tip of a field emission source is very high as a consequence of the small dimensions of the tip. This very high electrostatic field (xcx9c109 V/m) causes electrons to be emitted from the tip, which electrons then migrate to the target.
In inspection systems with cold field or Schottky emission electron sources, a false defect may be caused by random spikes in the electron beam current, i.e. emission noise. In field emission sources, emission noise is a serious problem. Such emission noise does not occur in thermionic and LaB6 electron emission sources; however thermionic emission sources have much lower brightness than field emission sources.
In field emission sources the effect of emission noise is independent from and additional to the effect of the beam dependent quantum shot noise. The quantum shot noise depends upon the total number of detected electrons, and its effect can be decreased by increasing the total beam current. Emission noise is caused by microscopic changes in the emission properties of a field emission or Schottky emitter tip, which results in a sudden change in the emitted beam current or a short current pulse. Although the emitter tip is placed in a high quality vacuum, one significant cause of such emission noise is the presence of residual gas. Gas molecules that remain may become ionized near the emitter tip. In turn such ionized gas molecules interact with the emitting surface of the emitter tip and give rise to random fluctuations in the beam current.
Typically a ZrO Schottky emitter, for example, may be subject to emission noise in the range of 1.5%, and this cannot be reduced by increasing the beam current. The emission noise may manifest itself in scanned electron images as an artificial defect. In imaging applications such as SEM (scanning electron microscopy) or metrology, these artificial defects can be averaged out by multiple pass averaging. However, multipass averaging is not desirable for inspection systems, since such averaging significantly increases the required inspection time and accordingly decreases throughput.
The purpose of the present invention is to reduce or eliminate the spurious effects of emission noise on the scanned electron images. The emission noise randomly increases or decreases the emitted electron beam and may manifest itself in scanned electron images as an artificial defect. The elimination of this noise both increases the detection sensitivity of an inspection tool and its throughput.
In one class of embodiments, the invention is an apparatus whose column configuration provides for emission noise reduction through the use of a beam-limiting element (having a beam-limiting aperture) for monitoring the electron beam current, and a screening element (having a screening aperture) positioned between the beam-limiting element and an electron source (emitters). The screening element collects most of the current transmitted from the emitter (e.g., most current transmitted by the first lens of the electron beam column). In order to achieve good noise suppression, the screening aperture should let through (to the beam-limiting element) only the portion of the beam where the electrons are correlated. The preferred implementation of this invention is the electron beam microcolumn; however the invention is also applicable to conventional columns operating at higher beam energies such as 10-100 kV.
In another class embodiments, the present invention is a method for emission noise reduction through the use of a screened beam-limiting aperture for monitoring the electron beam current. This novel method utilizes a screening aperture located between the emitter and the beam-limiting aperture, which screening aperture collects most of the current transmitted by the first lens of the electron beam column. In order to achieve good noise suppression, the screening aperture should transmit only the portion of the beam where the electrons are correlated.
According to the present invention, the current collected by the beam-limiting aperture is used as a reference signal in the image processing to correct for the effect of the emission noise. The elimination of noise (by processing the secondary electron data from the target using the reference signal) increases the detection sensitivity of an inspection tool. This reduces the total number of required pixels per substrate and therefore increases the throughput of the tool.